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The achievement of extensive and meaningful molecular dynamics simulations requires both the detailed knowledge of the basic features of the intermolecular interaction and the representation of the involved potential energy surface in a simple, natural and analytical form. This double request stimulated us to extend to ion-molecule systems a semiempirical method previously introduced for the description of weakly interacting atom-molecule aggregates and formulated in terms of atomic species-molecular bond interaction additivity. The method is here applied to the investigation of the prototypical M(+)-C6H6 systems (M = Li, Na, K, Rb and Cs) and some of its predictions are tested against accurate ab initio calculations. Such calculations have been performed by employing the MP2 method and large basis sets, privileging the description of the metal atoms. The agreement between potential energy scans semiempirically obtained and ab initio results is good for all the investigated geometries, thus showing that the adopted representation is in general able to reproduce all the main features of the potential energy surface for these systems. The role of the various noncovalent interaction components, as a function of the geometry and of the intermolecular distance in the M(+)-C6H6 complexes, is also investigated for a more detailed assessment of the results of the semiempirical method.

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We report a study on the behavior with total angular momentum J of several resonances occurring at collision energies below or slightly above the reaction barrier in the F+H2-->HF+H reaction. Resonance positions and widths are extracted from exact time-independent quantum mechanical calculations using the hyperquantization algorithm and Smith's Q-matrix formalism which exploits complete S-matrix information. The results confirm previous work but provide much greater insight. Identification of quasi-bound states responsible for the resonances based on adiabatic models for the long-range atom-molecule interactions both in the entrance and exit channels, is successful except for the feature occurring at the lowest energy, which is found to overlap with an exit-channel resonance for J approximately 7. The two features are analyzed as overlapping resonances and their excellent Lorentzian fits, with well-behaved J-dependences of positions and widths, support the interpretation of the low-energy feature as a resonance to be associated to the triatomic transition state of the reaction. Resonance role on the reactive observables (integral cross sections and angular distributions) is investigated. The mechanism leading to forward scattering in the reactive differential cross section is commented, while the effects on rate constants, as well as the sensitivity of the resonance pattern to modification of the potential energy surface, are fully discussed elsewhere.